1. Field of the Invention
This invention relates to an improvement in the treatment of Parkinson's disease and related disorders. More specifically, the present invention introduces novel formulations of the combination carbidopa and levodopa, the current mainstay of therapy.
2. Background and Prior Art
Parkinson's disease is associated with the depletion of dopamine from cells in the corpus striatum. Since dopamine does not cross the blood brain barrier and cannot therefore be used to treat Parkinson's disease, its immediate precursor, levodopa, is used instead because it penetrates the brain where it is decarboxylated to dopamine. But levodopa is also decarboxylated to dopamine in peripheral tissues and consequently only a small portion of administered levodopa is transported unchanged to the brain. This reaction can be blocked by carbidopa which inhibits decarboxylation of peripheral levodopa but cannot itself cross the blood brain barrier and has no effect on the metabolism of levodopa in the brain.
The combination of carbidopa and levodopa is considered to be the most effective treatment for symptoms of Parkinson's disease (The Medical Letter, 35:31-34, 1993). Nevertheless, certain limitations become apparent within two to five years of initiating combination therapy. As the disease progresses, the benefit from each dose becomes shorter ("the wearing off effect") and some patients fluctuate unpredictably between mobility and immobility ("the on-off effect"). "On" periods are usually associated with high plasma levodopa concentrations and often include abnormal involuntary movements, i.e., dyskinesias. "Off" periods have been correlated with low plasma levodopa and bradykinetic episodes.
In an effort to reduce the occurrences of "wearing off" and "on-off" phenomena, a controlled release oral dosage combination was introduced with claims of slow and simultaneous release of carbidopa and levodopa from the formulation (U.S. Pat. No. 4,900,755 issued Feb. 13, 1990). Data from clinical trials cited in the patent indicate that effective anti-Parkinson effects were achieved with fewer daily doses of the controlled release form as compared with the conventional combination.
Nevertheless, there remains a significant flaw in the therapeutic application of controlled release carbidopa-levodopa; that is, the considerable delay in onset of action. Mean time to peak concentration in healthy elderly subjects was found to be two hours for controlled release carbidopa-levodopa and only 0.5 hours for the conventional form (Physicians Desk Ref., .sub.47 th Ed., p. 976, 1993). A controlled release dosage form that could also provide rapid onset of action, at least equivalent to that of conventional carbidopa-levodopa would have an obvious clinical advantage over current therapy.
The strategy proposed in the present invention is to formulate oral dosage forms containing both immediate release carbidopa-levodopa and controlled release carbidopa-levodopa. Ingestion would provide rapid onset anti-Parkinson activity via the immediate release component followed by sustained therapeutic activity from the controlled release component.